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<title>Introducing magrittr</title>

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<h1 class="title toc-ignore">Introducing magrittr</h1>
<h4 class="author">Stefan Milton Bache</h4>
<h4 class="date">November, 2014</h4>



<div id="abstract" class="section level1">
<h1>Abstract</h1>
<p>The <em>magrittr</em> (to be pronounced with a sophisticated french accent) package has two aims: decrease development time and improve readability and maintainability of code. Or even shortr: make your code smokin’ (puff puff)!</p>
<p>To achieve its humble aims, <em>magrittr</em> (remember the accent) provides a new “pipe”-like operator, <code>%&gt;%</code>, with which you may pipe a value forward into an expression or function call; something along the lines of <code>x %&gt;% f</code>, rather than <code>f(x)</code>. This is not an unknown feature elsewhere; a prime example is the <code>|&gt;</code> operator used extensively in <code>F#</code> (to say the least) and indeed this – along with Unix pipes – served as a motivation for developing the magrittr package.</p>
<p>This vignette describes the main features of <em>magrittr</em> and demonstrates some features which have been added since the initial release.</p>
</div>
<div id="introduction-and-basics" class="section level1">
<h1>Introduction and basics</h1>
<p>At first encounter, you may wonder whether an operator such as <code>%&gt;%</code> can really be all that beneficial; but as you may notice, it semantically changes your code in a way that makes it more intuitive to both read and write.</p>
<p>Consider the following example, in which the <code>mtcars</code> dataset shipped with R is munged a little:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb1-1"><a href="#cb1-1"></a><span class="kw">library</span>(magrittr)</span>
<span id="cb1-2"><a href="#cb1-2"></a></span>
<span id="cb1-3"><a href="#cb1-3"></a>car_data &lt;-<span class="st"> </span></span>
<span id="cb1-4"><a href="#cb1-4"></a><span class="st">  </span>mtcars <span class="op">%&gt;%</span></span>
<span id="cb1-5"><a href="#cb1-5"></a><span class="st">  </span><span class="kw">subset</span>(hp <span class="op">&gt;</span><span class="st"> </span><span class="dv">100</span>) <span class="op">%&gt;%</span></span>
<span id="cb1-6"><a href="#cb1-6"></a><span class="st">  </span><span class="kw">aggregate</span>(. <span class="op">~</span><span class="st"> </span>cyl, <span class="dt">data =</span> ., <span class="dt">FUN =</span> . <span class="op">%&gt;%</span><span class="st"> </span>mean <span class="op">%&gt;%</span><span class="st"> </span><span class="kw">round</span>(<span class="dv">2</span>)) <span class="op">%&gt;%</span></span>
<span id="cb1-7"><a href="#cb1-7"></a><span class="st">  </span><span class="kw">transform</span>(<span class="dt">kpl =</span> mpg <span class="op">%&gt;%</span><span class="st"> </span><span class="kw">multiply_by</span>(<span class="fl">0.4251</span>)) <span class="op">%&gt;%</span></span>
<span id="cb1-8"><a href="#cb1-8"></a><span class="st">  </span>print</span>
<span id="cb1-9"><a href="#cb1-9"></a><span class="co">#&gt;   cyl   mpg   disp     hp drat   wt  qsec   vs   am gear carb       kpl</span></span>
<span id="cb1-10"><a href="#cb1-10"></a><span class="co">#&gt; 1   4 25.90 108.05 111.00 3.94 2.15 17.75 1.00 1.00 4.50 2.00 11.010090</span></span>
<span id="cb1-11"><a href="#cb1-11"></a><span class="co">#&gt; 2   6 19.74 183.31 122.29 3.59 3.12 17.98 0.57 0.43 3.86 3.43  8.391474</span></span>
<span id="cb1-12"><a href="#cb1-12"></a><span class="co">#&gt; 3   8 15.10 353.10 209.21 3.23 4.00 16.77 0.00 0.14 3.29 3.50  6.419010</span></span></code></pre></div>
<p>We start with a value, here <code>mtcars</code> (a <code>data.frame</code>). From there, we extract a subset, aggregate the information based on the number of cylinders, and then transform the dataset by adding a variable for kilometers per liter as a supplement to miles per gallon. Finally we print the result before assigning it. Note how the code is arranged in the logical order of how you think about the task: data-&gt;transform-&gt;aggregate, which is also the same order as the code will execute. It’s like a recipe – easy to read, easy to follow!</p>
<p>A horrific alternative would be to write:</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb2-1"><a href="#cb2-1"></a>car_data &lt;-<span class="st"> </span></span>
<span id="cb2-2"><a href="#cb2-2"></a><span class="st">  </span><span class="kw">transform</span>(<span class="kw">aggregate</span>(. <span class="op">~</span><span class="st"> </span>cyl, </span>
<span id="cb2-3"><a href="#cb2-3"></a>                      <span class="dt">data =</span> <span class="kw">subset</span>(mtcars, hp <span class="op">&gt;</span><span class="st"> </span><span class="dv">100</span>), </span>
<span id="cb2-4"><a href="#cb2-4"></a>                      <span class="dt">FUN =</span> <span class="cf">function</span>(x) <span class="kw">round</span>(<span class="kw">mean</span>(x), <span class="dv">2</span>)), </span>
<span id="cb2-5"><a href="#cb2-5"></a>            <span class="dt">kpl =</span> mpg<span class="op">*</span><span class="fl">0.4251</span>)</span></code></pre></div>
<p>There is a lot more clutter with parentheses, and the mental task of deciphering the code is more challenging—particularly if you did not write it yourself.</p>
<p>Note also how “building” a function on the fly for use in <code>aggregate</code> is very simple in <em>magrittr</em>: rather than an actual value as the left-hand side in the pipeline, just use the placeholder. This is also very useful in R’s <code>*apply</code> family of functions.</p>
<p>Granted, you may make the second example better, perhaps throw in a few temporary variables (which is often avoided to some degree when using <em>magrittr</em>), but one often sees cluttered lines like the ones presented.</p>
<p>And here is another selling point: suppose I want to quickly add another step somewhere in the process. This is very easy to do in the pipeline version, but a little more challenging in the “standard” example.</p>
<p>The combined example shows a few neat features of the pipe (which it is not):</p>
<ol style="list-style-type: decimal">
<li>By default the left-hand side (LHS) will be <em>piped in</em> as the first argument of the function appearing on the right-hand side (RHS). This is the case in the <code>subset</code> and <code>transform</code> expressions.</li>
<li><code>%&gt;%</code> may be used in a nested fashion, e.g. it may appear in expressions within arguments. This is illustrated in the <code>mpg</code> to <code>kpl</code> conversion.</li>
<li>When the LHS is needed at a position other than the first, one can use the dot,<code>&#39;.&#39;</code>, as placeholder. This is shown in the <code>aggregate</code> expression.</li>
<li>The dot in e.g. a formula is <em>not</em> confused with a placeholder, which is utilized in the <code>aggregate</code> expression.</li>
<li>Whenever only <em>one</em> argument (the LHS) is needed, one can omit the empty parentheses. This is shown in the call to <code>print</code> (which also returns its argument). Here, <code>LHS %&gt;% print()</code>, or even <code>LHS %&gt;% print(.)</code> would also work.</li>
<li>A pipeline with a dot (<code>.</code>) as the LHS will create a unary function. This is used to define the aggregator function.</li>
</ol>
<p>One feature, which was not demonstrated above is piping into <em>anonymous functions</em>, or <em>lambdas</em>. This is possible using standard function definitions, e.g.:</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb3-1"><a href="#cb3-1"></a>car_data <span class="op">%&gt;%</span></span>
<span id="cb3-2"><a href="#cb3-2"></a>(<span class="cf">function</span>(x) {</span>
<span id="cb3-3"><a href="#cb3-3"></a>  <span class="cf">if</span> (<span class="kw">nrow</span>(x) <span class="op">&gt;</span><span class="st"> </span><span class="dv">2</span>) </span>
<span id="cb3-4"><a href="#cb3-4"></a>    <span class="kw">rbind</span>(<span class="kw">head</span>(x, <span class="dv">1</span>), <span class="kw">tail</span>(x, <span class="dv">1</span>))</span>
<span id="cb3-5"><a href="#cb3-5"></a>  <span class="cf">else</span> x</span>
<span id="cb3-6"><a href="#cb3-6"></a>})</span></code></pre></div>
<p>However, <em>magrittr</em> also allows a short-hand notation:</p>
<div class="sourceCode" id="cb4"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb4-1"><a href="#cb4-1"></a>car_data <span class="op">%&gt;%</span></span>
<span id="cb4-2"><a href="#cb4-2"></a>{ </span>
<span id="cb4-3"><a href="#cb4-3"></a>  <span class="cf">if</span> (<span class="kw">nrow</span>(.) <span class="op">&gt;</span><span class="st"> </span><span class="dv">0</span>)</span>
<span id="cb4-4"><a href="#cb4-4"></a>    <span class="kw">rbind</span>(<span class="kw">head</span>(., <span class="dv">1</span>), <span class="kw">tail</span>(., <span class="dv">1</span>))</span>
<span id="cb4-5"><a href="#cb4-5"></a>  <span class="cf">else</span> .</span>
<span id="cb4-6"><a href="#cb4-6"></a>}</span>
<span id="cb4-7"><a href="#cb4-7"></a><span class="co">#&gt;   cyl  mpg   disp     hp drat   wt  qsec vs   am gear carb      kpl</span></span>
<span id="cb4-8"><a href="#cb4-8"></a><span class="co">#&gt; 1   4 25.9 108.05 111.00 3.94 2.15 17.75  1 1.00 4.50  2.0 11.01009</span></span>
<span id="cb4-9"><a href="#cb4-9"></a><span class="co">#&gt; 3   8 15.1 353.10 209.21 3.23 4.00 16.77  0 0.14 3.29  3.5  6.41901</span></span></code></pre></div>
<p>Since all right-hand sides are really “body expressions” of unary functions, this is only the natural extension of the simple right-hand side expressions. Of course, longer and more complex functions can be made using this approach.</p>
<p>In the first example, the anonymous function is enclosed in parentheses. Whenever you want to use a function- or call-generating statement as right-hand side, parentheses are used to evaluate the right-hand side before piping takes place.</p>
<p>Another, less useful example is:</p>
<div class="sourceCode" id="cb5"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb5-1"><a href="#cb5-1"></a><span class="dv">1</span><span class="op">:</span><span class="dv">10</span> <span class="op">%&gt;%</span><span class="st"> </span>(<span class="kw">substitute</span>(<span class="kw">f</span>(), <span class="kw">list</span>(<span class="dt">f =</span> sum)))</span>
<span id="cb5-2"><a href="#cb5-2"></a><span class="co">#&gt; [1] 55</span></span></code></pre></div>
</div>
<div id="additional-pipe-operators" class="section level1">
<h1>Additional pipe operators</h1>
<p><em>magrittr</em> also provides three related pipe operators. These are not as common as <code>%&gt;%</code> but they become useful in special cases.</p>
<p>The “tee” pipe, <code>%T&gt;%</code> works like <code>%&gt;%</code>, except it returns the left-hand side value, and not the result of the right-hand side operation. This is useful when a step in a pipeline is used for its side-effect (printing, plotting, logging, etc.). As an example (where the actual plot is omitted here):</p>
<div class="sourceCode" id="cb6"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb6-1"><a href="#cb6-1"></a><span class="kw">rnorm</span>(<span class="dv">200</span>) <span class="op">%&gt;%</span></span>
<span id="cb6-2"><a href="#cb6-2"></a><span class="kw">matrix</span>(<span class="dt">ncol =</span> <span class="dv">2</span>) <span class="op">%T&gt;%</span></span>
<span id="cb6-3"><a href="#cb6-3"></a>plot <span class="op">%&gt;%</span><span class="st"> </span><span class="co"># plot usually does not return anything. </span></span>
<span id="cb6-4"><a href="#cb6-4"></a>colSums</span>
<span id="cb6-5"><a href="#cb6-5"></a><span class="co">#&gt; [1] -6.159654  5.111522</span></span></code></pre></div>
<p>The “exposition” pipe, <code>%$%</code> exposes the names within the left-hand side object to the right-hand side expression. Essentially, it is a short-hand for using the <code>with</code> functions (and the same left-hand side objects are accepted). This operator is handy when functions do not themselves have a data argument, as for example <code>lm</code> and <code>aggregate</code> do. Here are a few examples as illustration:</p>
<div class="sourceCode" id="cb7"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb7-1"><a href="#cb7-1"></a>iris <span class="op">%&gt;%</span></span>
<span id="cb7-2"><a href="#cb7-2"></a><span class="st">  </span><span class="kw">subset</span>(Sepal.Length <span class="op">&gt;</span><span class="st"> </span><span class="kw">mean</span>(Sepal.Length)) <span class="op">%$%</span></span>
<span id="cb7-3"><a href="#cb7-3"></a><span class="st">  </span><span class="kw">cor</span>(Sepal.Length, Sepal.Width)</span>
<span id="cb7-4"><a href="#cb7-4"></a>   </span>
<span id="cb7-5"><a href="#cb7-5"></a><span class="kw">data.frame</span>(<span class="dt">z =</span> <span class="kw">rnorm</span>(<span class="dv">100</span>)) <span class="op">%$%</span><span class="st"> </span></span>
<span id="cb7-6"><a href="#cb7-6"></a><span class="st">  </span><span class="kw">ts.plot</span>(z)</span></code></pre></div>
<p>Finally, the “assignment” pipe <code>%&lt;&gt;%</code> can be used as the first pipe in a chain. The effect will be that the result of the pipeline is assigned to the left-hand side object, rather than returning the result as usual. It is essentially shorthand notation for expressions like <code>foo &lt;- foo %&gt;% bar %&gt;% baz</code>, which boils down to <code>foo %&lt;&gt;% bar %&gt;% baz</code>. Another example is:</p>
<div class="sourceCode" id="cb8"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb8-1"><a href="#cb8-1"></a>iris<span class="op">$</span>Sepal.Length <span class="op">%&lt;&gt;%</span><span class="st"> </span>sqrt</span></code></pre></div>
<p>The <code>%&lt;&gt;%</code> can be used whenever <code>expr &lt;- ...</code> makes sense, e.g. </p>
<ul>
<li><code>x %&lt;&gt;% foo %&gt;% bar</code></li>
<li><code>x[1:10] %&lt;&gt;% foo %&gt;% bar</code></li>
<li><code>x$baz %&lt;&gt;% foo %&gt;% bar</code></li>
</ul>
</div>
<div id="aliases" class="section level1">
<h1>Aliases</h1>
<p>In addition to the <code>%&gt;%</code>-operator, <em>magrittr</em> provides some aliases for other operators which make operations such as addition or multiplication fit well into the <em>magrittr</em>-syntax. As an example, consider:</p>
<div class="sourceCode" id="cb9"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb9-1"><a href="#cb9-1"></a><span class="kw">rnorm</span>(<span class="dv">1000</span>)    <span class="op">%&gt;%</span></span>
<span id="cb9-2"><a href="#cb9-2"></a><span class="kw">multiply_by</span>(<span class="dv">5</span>) <span class="op">%&gt;%</span></span>
<span id="cb9-3"><a href="#cb9-3"></a><span class="kw">add</span>(<span class="dv">5</span>)         <span class="op">%&gt;%</span></span>
<span id="cb9-4"><a href="#cb9-4"></a>{ </span>
<span id="cb9-5"><a href="#cb9-5"></a>   <span class="kw">cat</span>(<span class="st">&quot;Mean:&quot;</span>, <span class="kw">mean</span>(.), </span>
<span id="cb9-6"><a href="#cb9-6"></a>       <span class="st">&quot;Variance:&quot;</span>, <span class="kw">var</span>(.), <span class="st">&quot;</span><span class="ch">\n</span><span class="st">&quot;</span>)</span>
<span id="cb9-7"><a href="#cb9-7"></a>   <span class="kw">head</span>(.)</span>
<span id="cb9-8"><a href="#cb9-8"></a>}</span>
<span id="cb9-9"><a href="#cb9-9"></a><span class="co">#&gt; Mean: 4.811159 Variance: 23.07234</span></span>
<span id="cb9-10"><a href="#cb9-10"></a><span class="co">#&gt; [1] -2.221589  6.897375  6.868345  9.569161  3.470477 -1.358098</span></span></code></pre></div>
<p>which could be written in more compact form as:</p>
<div class="sourceCode" id="cb10"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb10-1"><a href="#cb10-1"></a><span class="kw">rnorm</span>(<span class="dv">100</span>) <span class="op">%&gt;%</span><span class="st"> `</span><span class="dt">*</span><span class="st">`</span>(<span class="dv">5</span>) <span class="op">%&gt;%</span><span class="st"> `</span><span class="dt">+</span><span class="st">`</span>(<span class="dv">5</span>) <span class="op">%&gt;%</span><span class="st"> </span></span>
<span id="cb10-2"><a href="#cb10-2"></a>{</span>
<span id="cb10-3"><a href="#cb10-3"></a>  <span class="kw">cat</span>(<span class="st">&quot;Mean:&quot;</span>, <span class="kw">mean</span>(.), <span class="st">&quot;Variance:&quot;</span>, <span class="kw">var</span>(.),  <span class="st">&quot;</span><span class="ch">\n</span><span class="st">&quot;</span>)</span>
<span id="cb10-4"><a href="#cb10-4"></a>  <span class="kw">head</span>(.)</span>
<span id="cb10-5"><a href="#cb10-5"></a>}</span></code></pre></div>
<p>To see a list of the aliases, execute e.g. <code>?multiply_by</code>.</p>
</div>
<div id="development" class="section level1">
<h1>Development</h1>
<p>The <em>magrittr</em> package is also available in a development version at the GitHub development page: <a href="https://github.com/tidyverse/magrittr">github.com/tidyverse/magrittr</a>.</p>
</div>



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